The present invention relates to a pressurized fluid operated mechanism, and in particular, relates to an integral hydraulically assisted steering gear for a vehicle.
A known integral power steering gear includes a housing defining a chamber. A piston is located within the chamber dividing the chamber into first and second chamber portions. The piston has a series of rack teeth formed thereon. The rack teeth mesh with a sector gear which is fixed to an output shaft. The output shaft is connected with a steering linkage connected with the steerable wheels of a vehicle to steer the vehicle when the output shaft rotates.
The housing further includes a fluid inlet port and a fluid outlet port. The inlet port is connected with the discharge passage of a hydraulic pump. The outlet port is connected with a return passage of the pump. A valve assembly including a pair of relatively rotatable valve elements is supported by the housing. The valve assembly controls the flow of pressurized fluid between the pump and one or the other of the first and second chambers. The valve assembly controls the direction and amount of steering. Pressurized fluid moves the piston and thereby rotated the output shaft. One of the valve elements is connected to a manually rotatable input shaft which is rotated by the manually operated steering wheel. The other of the valve members is connected with a follow-up member which rotates in response to movement of the piston.
Typically the follow-up member is a ball screw drive comprising a worm disposed in an axial bore formed in the piston. The worm and the bore in the piston are helically grooved to receive a plurality of balls therebetween. The grooves cooperate with balls to rotate the worm upon axial movement of the piston. Thus, the valve part connected with the follow-up member will rotate. In the event of a loss of fluid pressure, the follow-up member is rotated directly by the input shaft because of a mechanical connection between the input shaft and the follow-up member. Manual rotation of the follow-up member causes the balls to move the piston axially which rotates the output shaft to effect manual steering of the vehicle.
The known mechanism typically also includes a check valve located in the housing. The check valve allows circulation of fluid between the first and second chambers when the piston is moved manually such as during a steering maneuver occurring during a loss of fluid pressure. The location of the check valve in the housing has resulted in complications in manufacturing of the steering gear.
The fluid pressure which causes axial movement of the piston also results in a net axial load being applied to the follow-up member and/or valve member connected with the follow-up member. This occurs because the follow-up member is typically disposed within one of the chambers. Thus, the net axial load is transmitted to bearings which support the follow-up member and to the valve member connected with the follow-up member.